Imaging devices including solid state image sensors are being incorporated into low cost cameras and other handheld devices, such as mobile cellular telephones and personal digital assistants. A primary concern is to minimize the space utilized by the imaging device. It is also desirable to minimize the cost of the imaging device.
The image sensor typically includes a focussing lens for reproducing an image of the subject. The lens may be a single lens element or a compound lens formed by two or more elements. The lens may be formed from glass or a transparent plastic material.
Other separate components which are typically included in a lens assembly are an aperture, baffle and an infra-red filter. The aperture limits the quantity of light that reaches the lens by determining the diameter of light radiation. The position of the aperture is important since it is typically desirable to maximize the distance between the aperture and the image sensor. The aperture is typically provided by an opaque plastic material.
The baffle is provided to delimit the direction or angle of the light which passes through the aperture. The baffle typically requires a substantial thickness, typically around 0.3 mm, to have a suitable profile which can provide this delimiting function. The baffle is also typically formed from an opaque plastic material.
The infra-red filter is present to prevent infrared radiation impinging on the image sensor, and is provided on a glass plate, typically 0.3 mm thick. The glass plate and infra-red filter may be provided at a rear surface of the lens, between the lens and sensor, or they may be located near the aperture. In either position, the glass plate limits the space available for the lens. The infra-red filter comprises a number of coats, typically around 40 layers, of a suitable material provided on the glass plate. However, the overall thickness of the coating is still small, typically about 5 μm. The infra-red coating is typically not provided on the lens as the thermal co-efficient of expansion is too great and cracking of the coating would result.